1. Field of the Invention
The present invention relates to a light guide plate and a surface light source using the light guide plate, and more particularly, to a surface light source used for a back light of a liquid crystal display or the like, a lighting advertisement, a traffic-control sign, etc., and a light guide plate provided for the surface light source.
2. Description of the Prior Art
Examples of a surface light source for a back light of a liquid crystal display (LCD) include one of a side light type having a transparent flat plate as a light guide plate. In such a surface light source, light is incident from one of the side end surfaces of a light guide plate composed of a transparent parallel flat plate or a flat plate having a wedge shape in cross section, the light is propagated throughout the whole area of the light guide plate utilizing total reflection inside the transparent flat plate, a part of the propagated light is changed into diffused reflected light at less than a critical angle by a light scattering reflective plate on the reverse surface of the light guide plate, and diffused light is emitted from the surface of the light guide plate (see Japanese Utility Model Laid-Open No. 162201/1980, for example).
Furthermore, examples of a surface light source for a back light include one so adapted that a lens sheet having a projection of a triangular prism-type lenticular lens on its one surface, the other surface of which is smooth, is superimposed on the surface of a light guide plate provided for the above-mentioned surface light source with the surface of the projection directed upward, to diffuse its diffused emitted light uniformly and isotropically in a desired angular range utilizing a light focusing function of the lens (see, Japanese Utility Model Laid-Open No. 107201/1992, for example).
When the lens sheet is used in combination with a mat transparent diffusing plate (a mat transparent sheet), light energy of a light source is more intensively distributed in a desired restricted angular range, and diffused light higher in uniformity and isotropy can be obtained in the angular range, as compared with a light guide plate using only the mat transparent diffusing plate as disclosed in U.S. Pat. No. 4,729,067.
Furthermore, as a measure to achieve higher luminance of a back light used for a liquid crystal display, there has recently been a tendency to employ such construction that two lenticular lens sheets are overlapped with each other at right angles (Monthly Display, May 1996, pp. 35-pp. 39). This type of surface light source is shown in FIG. 15. A light source 21 comprising a cold-cathode tube 21a and a reflector 21b is arranged on one of side end surfaces of a light guide plate 20 composed of a transparent flat plate having a ridge shape in cross section. Light is incident on the light guide plate 20 from the light source 21. The light is propagated throughout the whole area of the light guide plate 20 utilizing total reflection inside the transparent flat plate, a part of the propagated light is taken as diffused reflected light at less than a critical angle by a light scattering reflective plate 22 on the reverse surface of the light guide plate 20, and the diffused light is emitted from a diffusing plate 23 provided on a front surface of the light guide plate 20. Lenticular lens sheets 24 and 25 each having a projection of a triangular prism-type lenticular lens on its one surface, the other surface of which is smooth, are overlapped with each other at right angles, and its diffused emitted light is uniformly and isotropically diffused in a desired angular range utilizing a light focusing function of the lens.
In a surface light source for a back light in a conventional liquid crystal display, there is not only a problem that visual characteristics are relatively narrow but also a problem with high cost of parts because two lenticular lens sheets are used, and huge fabrication costs because the two lenticular lens sheets must be mounted and aligned at right angles, so that the cost of the surface light source for a back light itself is significantly increased. Research and development of a surface light source for a back light in which the necessity of lenticular lens sheets, light diffusion dot pattern printing on the reverse surface of a diffusing plate or a light guide plate, and the like is eliminated has been reported (The Institute of Electronics, Information and Communication Engineers, Technical Report, EID98-79, pp. 67-pp. 72).